1. Field of the Invention
The present invention relates to systems and methods for controlling spacecraft, and in particular to a system and method for reducing instrument-induced spacecraft attitude jitter.
2. Description of the Related Art
Spacecraft jitter is a common problem experienced by fine-pointing Earth or space observation spacecraft. Spacecraft jitter may be caused by a number of factors, including the motion of instruments mounted in or on the spacecraft.
Conventional solutions to reduce spacecraft jitter fall into two categories. The first solution is to increase spacecraft attitude control system bandwidth. This provides tighter control over the spacecraft attitude, thus reducing the peak and average jitter. Unfortunately, attitude control system bandwidth is usually limited by control system stability concerns and can not be significantly increased. Further, increased bandwidth may also result in increased fuel usage and shortened operating lifetimes.
Another solution is to improve spacecraft structural damping using high damping materials or active damping devices. Unfortunately, this solution tends to significantly increase the cost of the spacecraft.
Still another solution is to use feedforward compensation to enhance the bandwidth limit of a conventional spacecraft control system. This solution is illustrated in U.S. Pat. No. 5,517,418, issued May 14, 1996 to Green et al. and entitled xe2x80x9cSpacecraft Disturbance Compensation Using Feedforward Control,xe2x80x9d and is hereby incorporated by reference herein. This patent illustrates the employment of feedforward spacecraft compensation to account for spacecraft thermal shock. Unfortunately, however, while the solution employed by the Green reference is effective in controlling spacecraft attitude in the presence of low-bandwidth disturbance sources such as thermal shock, it is less effective in reducing the high frequency harmonic jitter such as those that are induced by interaction of spacecraft instrument scanning and the structural vibration modes of the spacecraft. What is needed is a solution that reduces spacecraft jitter from these higher bandwidth sources. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a method, apparatus, article of manufacture, and a memory structure for reducing jitter induced by interaction of the scanning of on-board spacecraft instruments and spacecraft structural oscillation modes.
The apparatus comprises a spacecraft control processor for producing an actuator command signal, a signal generator, for producing a cancellation signal having at least one harmonic having a frequency and an amplitude substantially equal to that of a disturbance harmonic interacting with a spacecraft structural resonance and a phase substantially out of phase with the disturbance harmonic interacting with the spacecraft structural resonance, and at least one spacecraft control actuator, communicatively coupled to the spacecraft control processor and the signal generator for inducing satellite motion according to the actuator command signal and the cancellation signal.
The method comprises the steps of generating a cancellation signal having at least one harmonic having a frequency and an amplitude substantially equal to that of a disturbance harmonic interacting with a spacecraft structural resonance and a phase substantially out of phase with the disturbance harmonic interacting with the spacecraft structural resonance, and providing the cancellation signal to a spacecraft control actuator. The apparatus comprises a storage device tangibly embodying the method steps described above.
The present invention uses feedforward disturbance compensation to reduce jitter from interaction between instrument scanning and spacecraft structural vibration. This is accomplished by characterizing the scan disturbance by its harmonics that interact with spacecraft structures. This allows the interaction problem between spacecraft instrument scan and its structural vibration to be completely described by magnitudes and phases of a small number (typically one or two) of harmonics of the scan disturbance.
Predictions of the magnitudes and phases of these disturbance harmonics can be obtained by ground tests of the instrument, or may be determined in-orbit after the satellite has been deployed. Repeated tests can provide accurate prediction of these magnitudes and phases and of their variations with respect to the instrument scan frequency and/or scan pattern.
Using these predictions and/or measurements, the present invention generates a periodic cancellation signal that includes the same harmonics of equal magnitude but opposite phase to cancel the scan disturbance harmonics that interact with the structures.
In one embodiment, the present invention synchronizes the cancellation system using the instrument scan command. This provides an accurate timing for the cancellation system. In another embodiment, the present invention uses the estimates of spacecraft jitter magnitude and zero-crossing to synchronize the cancellation signal when the scan command is not available.
The foregoing provides the spacecraft control system with a jitter reduction capability that is not achievable by conventional spacecraft control methods. A conventional spacecraft control system usually has a limited control bandwidth because of its control time delay and its interaction with spacecraft flexibility. The control system has certain jitter reduction capability within its bandwidth, but can not provide any jitter reduction outside its bandwidth.
The multi-frequency cancellation system is also capable of reducing spacecraft jitter in a much wider frequency range. Its bandwidth equals to half of its system sample rate that is, in most of the cases, more than ten times wider than the bandwidth of a conventional spacecraft control system. With a good prediction of jitter frequencies and phases the cancellation system can achieve much better jitter reduction even within the bandwidth of the conventional control system. The present invention can completely cancel the jitter if disturbance magnitude, jitter frequency and phase are exactly predicted. The cancellation system can also be advantageously used in harmony with a conventional control system to form a highly effective spacecraft jitter control system.
In most cases, implementation of the present invention is simplified because only a few disturbance harmonics (one or two) will interact with spacecraft structures and the generated cancellation signal need only contain one or two sinusoidal signal components.
The magnitudes, phases and frequencies of the instrument scan disturbance can usually be accurately predicted. The magnitudes can be obtained by repeated ground tests of the instrument. The frequencies and phases can be derived from the instrument scan start-time and scan pattern. Even when the scan start-time and scan pattern are not available, estimating the magnitude and zero-crossing of spacecraft attitude jitter is not difficult, and can be accomplished with simple algorithms. With estimated jitter zero-crossing time, the frequency and phase are readily derived. Estimated jitter magnitude can then be used to set the state of the cancellation on/off signal.